Honeycomb filter

ABSTRACT

The honeycomb filter includes a honeycomb substrate having porous partition walls defining a plurality of cells and plugging portions. In a cross section, the cells are arranged so that a periphery of an inlet plugged cell including an open end whose shape is a square in which a length of one side is L 1  is surrounded with four rectangular outlet plugged cells each including an open end whose shape is a rectangle in which a length of one side is L 1  and a length of the other side is L 2  (L 1 &gt;L 2 ) and four square outlet plugged cells each including an open end whose shape is a square in which a length of one side is L 2 . A partition wall center distance a, a partition wall center distance b and a partition wall thickness t satisfy the following equation ( 1 ): 
         0.95&lt;   b/at&lt;   1.90    (1).

The present application is an application based on JP 2014-180013 filedon Sep. 4, 2014 with Japan Patent Office, the entire contents of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a honeycomb filter. More particularly,it relates to a honeycomb filter to be suitably used in purification ofparticulate matter included in an exhaust gas from an engine, especiallya car engine, and toxic gas components such as nitrogen oxides (NOx),carbon monoxide (CO) and hydrocarbons (HC).

2. Description of the Related Art

In recent years, reductions of fuel consumptions of cars have beenrequired from the viewpoints of influences on the global environment andresource saving. Consequently, there is the tendency that internalcombustion engines such a direct injection type gasoline engine and adiesel engine each having an excellent thermal efficiency are used aspower sources for the cars.

On the other hand, in these internal combustion engines, generation ofashes generated during combustion of fuel raises problems. From theviewpoint of the air atmosphere, there are required countermeasures forremoval of toxic components included in the exhaust gas andsimultaneously, prevention of emission of particulate matter(hereinafter referred to as “the PM” sometimes) such as soot or ash tothe atmosphere.

In particular, there is the tendency that regulations on the removal ofthe PM emitted from the diesel engine are strengthened worldwide, anduse of a honeycomb filter as a trapping filter to remove the PM(hereinafter referred to as “the DPF” sometimes) attracts attention.Further, there have been suggested various exhaust gas purificationsystems in which such a honeycomb filter is used. The above DPF usuallyhas a structure where a plurality of cells which become through channelsfor a fluid are usually defined by porous partition walls and cells arealternately plugged, whereby the porous partition walls constituting thecells perform a role of the filter.

In the DPF, the exhaust gas containing the particulate matter and thelike flow in the DPF from a first end face (an inflow end face), theparticulate matter is filtered with the partition walls, and then, thepurified gas flows out from a second end face (an outflow end face). Insuch a DPF, there has been the problem that, with the inflow of theexhaust gas, the particulate matter contained in the exhaust gas isdeposited on the partition walls to close the cells on an exhaust gasinflow side. This is a phenomenon which easily occurs in a case where alarge amount of the particulate matter is contained in the exhaust gas,or in cold districts. When the cells are closed in this manner, theproblem occurs that a pressure loss in the DPF rapidly enlarges.Therefore, to inhibit such cell closing, the filter is contrived toincrease a filtration area or an open frontal area in the exhaust gasinflow side cells.

As the above structure of the DPF, specifically, a structure has beensuggested in which a sectional area of each inflow side cell isdifferent from a sectional area of each outflow side cell (e.g., seePatent Document 1). Here, the sectional area of each cell indicates thearea of the cell in a cross section when the cell is cut along a planeperpendicular to a central axis direction of the cell. The inflow sidecells are cells which are opened in the inflow end face and whose openends in the outflow end face are plugged by plugging portions, and thecells are also referred to as outlet plugged cells. On the other hand,the outflow side cells are cells whose open ends in the inflow end faceare plugged by the plugging portions and which are opened in the outflowend face, and the cells are also referred to as inlet plugged cells.Additionally, hereinafter, the structure in which the sectional area ofeach inflow side cell is different from the sectional area of eachoutflow side cell will be referred to as “the HAC structure” sometimes.“The HAC” is abbreviation for High Ash Capacity.

In addition, there has been suggested a honeycomb filter of the HACstructure which has inflow side cells whose sectional areas are largeand outflow side cells whose sectional areas are small and in which asectional shape of each inflow side cell is different from a sectionalshape of each outflow side cell (e.g., see Patent Document 2). Here, thesectional shape of the cell is a shape which appears in a cross sectionof the cell when the cell is cut along a plane perpendicular to acentral axis direction of the cell.

In addition, as another structure of the DPF, a structure has beensuggested in which a periphery of a trapping cell group constituted of aplurality of inflow side cells (outlet plugged cells) is surrounded witha plurality of outflow side cells (inlet plugged cells) (e.g., PatentDocument 3).

To use the honeycomb filter continuously for a long period of time, itis necessary to periodically subject the honeycomb filter to aregeneration treatment. That is, for the purpose of reducing thepressure loss enlarged due to the soot deposited in the honeycomb filterwith an elapse of time to return a filter performance to an initialstate, it is necessary to burn and remove the soot deposited in thehoneycomb filter by a high-temperature gas. Hereinafter, the burning andremoval of the soot deposited in the honeycomb filter will simply bereferred to as “regeneration” of the honeycomb filter sometimes.

[Patent Document 1] WO2009/069378

[Patent Document 2] JP-A-2004-000896

[Patent Document 3] JP-A-2010-053697

SUMMARY OF THE INVENTION

However, heightening of an open frontal area of inflow side cells(outlet plugged cells) relatively results in decrease of an open frontalarea of outflow side cells (inlet plugged cells), and hence, there hasbeen the problem that, with this decrease, a pressure loss in an initialstage (the initial pressure loss) disadvantageously heightens.

Furthermore, the decrease of the open frontal area of the outflow sidecells (the inlet plugged cells) results in increase of a heat capacityon an inflow end face side due to plugging portions disposed on theinflow end face side of a DPF. In consequence, there has been theproblem that temperature rise on the inflow end face side due to anexhaust gas becomes dull and continuous regeneration properties due toNO₂ worsen.

In addition, when inflow side cells (the outlet plugged cells) differsfrom the outflow side cells (the inlet plugged cells) in a sectionalarea and a sectional shape, a thickness of partition walls forming thecells becomes smaller in a part of a portion in which the partitionwalls intersect each other (hereinafter referred to as “the intersectingportion” sometimes) in a certain case, and there has been the problemthat strength deteriorates. Consequently, there has been the problemthat, when PM deposited on the DPF is burnt and removed by postinjection, thermal stress is concentrated on a part of the thinnedintersecting portion, and breakage such as generation of cracks iseasily caused. Here, the portion (the intersecting portion) in which thepartition walls intersect each other is a portion belonging to both ofthe partition walls intersecting each other in the cross section of ahoneycomb filter such as the DPF cut along a plane perpendicular to acentral axis direction of the filter. For example, when the linearlyextending partition walls having the same thickness intersect eachother, the intersecting portion in the above cross section is a range ofa square sectional shape in the portion in which the partition wallsintersect.

In a structure of a honeycomb filter described in Patent Document 3, aperiphery of a trapping cell group constituted of a plurality of inflowside cells (outlet plugged cells) is surrounded with a plurality ofoutflow side cells (inlet plugged cells), and hence, PM in the honeycombfilter can be prevented from being rapidly burnt. Consequently, when thePM deposited on the honeycomb filter is burnt and removed, thermalstress is concentrated on a part of the honeycomb filter, and thegeneration of the cracks can be inhibited. However, in this structure ofthe honeycomb filter described in Patent Document 3, there has been theproblem that, when the plurality of inflow side cells (the outletplugged cells) are arranged discontinuously from one another, heatduring the burning and removal of the PM is hard to be propagated to theadjacent inflow side cell (the outlet plugged cell), and the honeycombfilter cannot efficiently be regenerated.

The present invention has been developed in view of such problems ofconventional technologies. According to the present invention, there isprovided a honeycomb filter in which both of a pressure loss in aninitial stage and a pressure loss during PM deposition are minimized, anefficiency of regeneration of the honeycomb filter during PM burningimproves, and further, generation of cracks due to thermal stress isdecreased.

The present inventors have found that the above problem in the initialpressure loss and the pressure loss during the PM deposition and theabove problem in the generation of the cracks can be solved by arranginginlet plugged cells and outlet plugged cells having predetermined shapesat predetermined positions. Specifically, there is provided a structurewhere an inlet plugged cell whose open end has a square shape in a crosssection perpendicular to a central axis direction of the honeycombfilter is surrounded with eight outlet plugged cells. In addition, thepresent inventors have found that for the purpose of improving theregeneration efficiency of the honeycomb filter while inhibiting thegeneration of the cracks, the plurality of outlet plugged cells do nothave to be arranged discontinuously from one another as described inPatent Document 3, and conversely, it is effective to continuouslyarrange the cells. Consequently, the present inventors have completedthe present invention. That is, according to the present invention, thefollowing honeycomb filter is provided.

[1] A honeycomb filter including a honeycomb substrate having porouspartition walls defining a plurality of cells which extend from aninflow end face as an end face on an exhaust gas inflow side to anoutflow end face as an end face on an exhaust gas outflow side and whichbecome through channels for a fluid; and plugging portions disposed inend portions of the plurality of cells on one of an inflow end face sideand an outflow end face side, wherein the cells of part of the pluralityof cells are inlet plugged cells whose end portions are closed by theplugging portions on the inflow end face side of the honeycombsubstrate, and the residual cells among the plurality of cells areoutlet plugged cells whose end portions are closed by the pluggingportions on the outflow end face side of the honeycomb substrate,wherein the plurality of cells are arranged so that a periphery of oneof the inlet plugged cells is surrounded with eight of the outletplugged cells in a cross section perpendicular to a central axisdirection of the honeycomb substrate, a shape of an open end of each ofthe inlet plugged cells in the cross section is a square in which alength of one side is L1, the outlet plugged cells include square outletplugged cells and rectangular outlet plugged cells, a shape of an openend of each of the square outlet plugged cells in the cross section is asquare in which a length of one side is L2, and L2 is smaller than L1,and a shape of an open end of each of the rectangular outlet pluggedcells in the cross section is a rectangle in which a length of a longside is L1 and a length of a short side is L2, and in each diagonaldirection of the square in which the length of the one side is L1 in thecross section, four of the square outlet plugged cells are arrangedadjacent to the inlet plugged cell, and in a linear directionperpendicular to each side of the square in which the length of the oneside is L1 in the cross section, four of the rectangular outlet pluggedcells are arranged so that the cells are adjacent to the inlet pluggedcell and so that the long side of the rectangle in the cross section isparallel to one side of the square in which the length of the one sideis L1 in the cross section, a partition wall thickness of the partitionwalls is defined as t, a distance from an intermediate point of thethickness of the partition wall defining one side of the open end of thesquare in which the length of the one side is L1 in the cross section toan intermediate point of the thickness of the partition wall defining aside facing the one side is defined as a partition wall center distancea, a distance from an intermediate point of the thickness of thepartition wall defining a long side of the open end of the rectangle inthe cross section to an intermediate point of the thickness of thepartition wall defining a side facing the long side is defined as apartition wall center distance b, and the partition wall center distancea, the partition wall center distance b and the partition wall thicknesst satisfy the following equation (1),

0.95<b/at<1.90   (1).

[2] The honeycomb filter according to the above [1], wherein thepartition wall center distance a is 1.4 mm or more and 2.4 mm or less.

[3] The honeycomb filter according to the above [1] or [2], wherein thepartition wall center distance b is 0.22 mm or more and 1.08 mm or less.

[4] The honeycomb filter according to any one of the above [1] to [3],wherein the partition wall thickness t is 0.16 mm or more and 0.34 mm orless.

[5] The honeycomb filter according to any one of the above [1] to [4],wherein a cell density of the honeycomb substrate is from 100 to 650cells/cm².

[6] The honeycomb filter according to any one of the above [1] to [5],wherein an open frontal area of the outlet plugged cells on the inflowend face side is from 13 to 50%.

[7] The honeycomb filter according to any one of the above [1] to [6],wherein a porosity of the partition walls is from 28 to 70%.

The honeycomb filter of the present invention is capable of efficientlytrapping and removing particulate matter included in an exhaust gasemitted from a direct injection type gasoline engine or a diesel engine,and reducing pressure loss in an initial stage as well as during PMdeposition. Additionally, the honeycomb filter of the present inventionis capable of effectively preventing generation of cracks due toconcentration of thermal stress during PM burning, and the like. Inaddition, heat during the PM burning is easily propagated among aplurality of inflow side cells (outlet plugged cells) and hence,regeneration of the honeycomb filter can efficiently be performed.Furthermore, the honeycomb filter of the present invention has lessplugging portions on an inflow end face side, and hence, a heat capacityon the inflow end face side of the honeycomb filter becomes smaller,ignitability during the PM burning improves, and continuous regenerationproperties by NO₂ improve.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing one embodiment of ahoneycomb filter of the present invention;

FIG. 2 is a partially enlarged view of the honeycomb filter shown inFIG. 1 and seen from an inflow end face side;

FIG. 3 is a partially enlarged view of the honeycomb filter shown inFIG. 1 and seen from an outflow end face side;

FIG. 4 is a view showing a cross section in an A-A′ direction shown inFIG. 2; and

FIG. 5 is a schematic partially enlarged view of a conventionalhoneycomb filter seen from an inflow end face side.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings. The present invention is not limited to thefollowing embodiments, and changes, modifications and improvements canbe added without departing from the gist of the invention.

A honeycomb filter of one embodiment of a honeycomb filter of thepresent invention is such a honeycomb filter 100 as shown in FIG. 1 toFIG. 4. The honeycomb filter 100 of the present embodiment includes ahoneycomb substrate 7 and plugging portions 3. The honeycomb substrate 7has the porous partition walls 1 defining a plurality of cells 2 whichextend through the honeycomb substrate from a first end face (an inflowend face 5 a) to a second end face (an outflow end face 5 b) and whichbecome through channels for an exhaust gas. The plugging portions 3 aredisposed in end portions of cells of part of the plurality of cells 2 onthe side of the first end face (the inflow end face 5 a) and endportions of the residual cells among the plurality of cells 2 on theside of the second end face (the outflow end face 5 b). FIG. 1 is aschematic perspective view showing one embodiment of the honeycombfilter of the present invention. FIG. 2 is a partially enlarged view ofthe honeycomb filter shown in FIG. 1 and seen from the inflow end faceside. FIG. 3 is a partially enlarged view of the honeycomb filter shownin FIG. 1 and seen from the outflow end face side. FIG. 4 is a viewshowing a cross section (an A-A′ cross section) in an A-A′ directionshown in FIG. 2. It is to be noted that the A-A′ cross sections in FIG.2 and FIG. 3 are the same cross sections.

As shown in FIG. 1 to FIG. 4, the plurality of cells 2 are constitutedof rectangular outlet plugged cells 2 a, square outlet plugged cells 2 band inlet plugged cells 2 c. The rectangular outlet plugged cells 2 aand the square outlet plugged cells 2 b are the cells 2 which are openedin the inflow end face 5 a for a fluid and in which outflow sideplugging portions 3 b are disposed in end portions on the side of theoutflow end face 5 b for the fluid. The inlet plugged cells 2 c are thecells 2 in which inflow side plugging portions 3 a are disposed in endportions on the side of the inflow end face 5 a and which are opened inthe outflow end face 5 b.

As shown in FIG. 1 to FIG. 4, in the honeycomb filter 100 of the presentembodiment, the plurality of cells 2 are arranged so that a periphery ofone of the inlet plugged cells 2 c is surrounded with eight outletplugged cells in a cross section perpendicular to a central axisdirection of the honeycomb substrate 7. In addition, a shape of an openend of each of the inlet plugged cells 2 c in the cross section is asquare in which a length of one side is L1 (hereinafter referred to alsoas “the length L1”). The outlet plugged cells include the square outletplugged cells 2 b and rectangular outlet plugged cells 2 a. A shape ofan open end of each of the square outlet plugged cells 2 b in the crosssection is a square in which a length of one side is L2 (hereinafterreferred to also as “the length L2”), and the length L2 is smaller thanthe length L1. In addition, a shape of an open end of each of therectangular outlet plugged cells 2 a in the cross section is a rectanglein which a length of a long side is L1 and a length of a short side isL2. Additionally, in the honeycomb filter 100 of the present embodiment,four of the square outlet plugged cells 2 b are arranged adjacent to theinlet plugged cell 2 c in each diagonal direction of the square in whichthe length of the one side is L1 in the cross section. Further, in alinear direction perpendicular to each side of the square in which thelength of the one side is L1 in the cross section, four of therectangular outlet plugged cells 2 a are adjacent to the inlet pluggedcell 2 c. In this case, the cells are arranged so that the long side ofthe rectangle of the rectangular outlet plugged cell 2 a in the crosssection is parallel to one side of the inlet plugged cell 2 c in thecross section (one side of the square in which the length of the oneside is L1). Here, a partition wall thickness of the porous partitionwalls 1 is defined as t, and a distance from an intermediate point ofthe thickness of the partition wall 1 defining one side of the open endof the square in which the length of the one side is L1 in the crosssection to an intermediate point of the thickness of the partition wall1 defining a side facing the one side is defined as a partition wallcenter distance a. In addition, a distance from an intermediate point ofthe thickness of the partition wall 1 defining a long side of the openend of the rectangle in the cross section to an intermediate point ofthe thickness of the partition wall 1 defining a side facing the longside is defined as a partition wall center distance b. Further, in thehoneycomb filter 100 of the present embodiment, the partition wallcenter distance a, the partition wall center distance b and thepartition wall thickness t satisfy the following equation (1):

0.95<b/at<1.90   (1).

The honeycomb filter 100 of the present embodiment has such aconstitution as described above, thereby producing the effects that atrapping efficiency of particulate matter in the exhaust gas is high,pressure losses in an initial stage of use and during particulate matterdeposition are low and the honeycomb filter is excellent in heatresistance strength and can efficiently be regenerated.

In the present embodiment, there is not any special restriction on amaterial constituting the honeycomb substrate 7. From the viewpoints ofstrength, heat resistance, durability and the like, main components ofthe material constituting the honeycomb substrate 7 are preferablyvarious ceramics of oxides or non-oxides, metals and the like. It isconsidered that examples of the ceramics include cordierite, mullite,alumina, spinel, silicon carbide, silicon nitride and aluminum titanate.In addition, it is considered that examples of the metal include anFe—Cr—Al series metal and metal silicon. At least one or at least twoselected from the group consisting of these materials is preferably themain component of the material constituting the honeycomb substrate 7.From the viewpoints of high strength, high heat resistance and the like,at least one or at least two selected from the group consisting ofalumina, mullite, aluminum titanate, cordierite, silicon carbide andsilicon nitride is especially preferably the main component of thematerial constituting the honeycomb substrate 7. In addition, from theviewpoints of high thermal conductivity, high heat resistance and thelike, silicon carbide or a silicon-silicon carbide composite material isespecially suitable as the main component of the material constitutingthe honeycomb substrate 7. Here, “the main component” means that a massratio of the component in a mass of the honeycomb substrate 7 is 50 mass% or more, preferably 70 mass % or more and further preferably 80 mass %or more.

In addition, a porosity of the partition walls 1 of the honeycombsubstrate 7 is preferably from 28 to 70%, further preferably from 30 to70%, and especially preferably from 35 to 68%. It is to be noted thatthe porosity of the partition walls is a value measured by mercuryporosimetry.

In the present embodiment, there is not any special restriction on amaterial of the plugging portions 3 as well. The material of theplugging portions 3 preferably includes at least one or at least twoselected from the group consisting of various ceramics, metals and thelike that are the above mentioned preferable examples of the material ofthe honeycomb substrate 7.

In addition, a porosity of the plugging portions 3 is preferably from 28to 70%, further preferably from 30 to 70%, and especially preferablyfrom 35 to 68%. When thermal expansion during PM burning is taken intoconsideration, in the honeycomb substrate 7, each of the porosity of thepartition walls 1 and the porosity of the plugging portions 3 ispreferably from 35 to 68%. It is to be noted that the porosity of theplugging portions is a value measured by the mercury porosimetry.

A cell density of the honeycomb filter 100 (the number of the cells 2per unit volume in a cross section perpendicular to a central axis ofthe honeycomb filter 100) is preferably from 100 to 650 cells/cm²,further preferably from 200 to 650 cells/cm², and especially preferablyfrom 300 to 600 cells/cm². When the cell density is smaller than 100cells/cm², a trapping performance deteriorates in a certain case. Whenthe cell density is larger than 650 cells/cm², the PM is deposited inthe vicinity of the inflow end face 5 a of the honeycomb filter 100, andthe cells 2 are gradually closed by the PM, so that pressure lossenlarges in a certain case.

In addition, although not shown in the drawings, the honeycomb substrateconstituting the honeycomb filter may be a honeycomb substrate of asegmented structure. Specifically, an example of the honeycomb substrateof the segmented structure is a honeycomb substrate in which a pluralityof honeycomb segments are bonded in a state where the honeycomb segmentsare arranged adjacent to each other so that side surfaces of thehoneycomb segments face each other. Each honeycomb segment has porouspartition walls defining a plurality of cells which extend from aninflow end face to an outflow end face and become through channels for afluid, and an outer wall disposed to surround the partition walls. Acircumferential wall is disposed in an outermost circumference of abonded body in which the plurality of honeycomb segments are bonded. Inaddition, a circumferential portion of the bonded body in which theplurality of honeycomb segments are bonded is processed by grinding orthe like, and a cross section perpendicular to a cell extendingdirection is formed into a round shape or the like. Afterward, thecircumferential wall may be disposed by applying a ceramic material tothe outermost circumference. Such a so-called honeycomb substrate of thesegmented structure is usable in place of a so-called monolithichoneycomb substrate shown in FIG. 1. Additionally, for the honeycombsubstrate constituting the honeycomb filter, slits may be formed in apart of the honeycomb substrate. In the honeycomb substrate of thesegmented structure or the honeycomb filter in which the honeycombsubstrate formed with the slits is used, thermal stress to be applied tothe filter can be dispersed, and generation of cracks due to localtemperature rise can be prevented.

There is not any special restriction on a size or a shape of eachsegment in a case where the plurality of honeycomb segments areintegrated. However, when each segment is excessively large, a crackpreventing effect by segmentation cannot sufficiently be exerted, andwhen each segment is excessively small, manufacturing of each segment orthe integration by the bonding becomes laborious in a certain case.There is not any special restriction on the shape of such a honeycombsegment, and for example, a quadrangular sectional shape, i.e., aquadrangular pillar-shaped segment is defined as a basic shape, and acircumferential shape of the monolithic honeycomb filter canappropriately be selected and processed.

There is not any special restriction on the whole shape of the honeycombfilter of the present embodiment, and in addition to such a roundsectional shape as shown in FIG. 1, examples of the sectional shapeinclude substantially round shapes such as an elliptic shape, a racetrack shape and an oblong shape and polygonal shapes such as aquadrangular shape and a hexagonal shape.

The honeycomb filter of the present embodiment will be described in moredetail with reference to the drawings. As shown in FIG. 2 and FIG. 3, inthe honeycomb filter 100 of the present embodiment, the shape of theopen end of each of the inlet plugged cells 2 c in the cross sectionperpendicular to the central axis direction of the honeycomb substrate 7is the square in which the length of one side is L1. Additionally, asshown in FIG. 2 and FIG. 3, in the honeycomb filter 100 of the presentembodiment, the shape of the open end of each of the square outletplugged cells 2 b in the cross section perpendicular to the central axisdirection of the honeycomb substrate 7 is the square in which the lengthof one side is L2, and L2 is smaller than L1. In addition, as shown inFIG. 2 and FIG. 3, the shape of the open end of each of the rectangularoutlet plugged cells 2 a in the cross section perpendicular to thecentral axis of the honeycomb substrate 7 is a rectangle in which alength of a long side is L1 and a length of a short side is L2. FIG. 5is a schematic partially enlarged view of a conventional honeycombfilter seen from an inflow end face side. In a honeycomb filter 200shown in FIG. 5, inlet plugged cells 202 b and 202 c and outlet pluggedcells 202 a are arranged in a checkered manner. It is to be noted that,in FIG. 5, reference numeral 201 indicates a partition wall. In FIG. 5,reference numeral 202 indicates a cell. In FIG. 5, reference numeral 203indicates a plugging portion. In FIG. 5, reference numeral 205 aindicates an inflow end face. On the other hand, as shown in FIG. 1 toFIG. 4, the honeycomb filter 100 of the present embodiment has theconstitution where the inlet plugged cell 2 c is surrounded with foursquare outlet plugged cells 2 b and four rectangular outlet pluggedcells 2 a. Consequently, in the honeycomb filter 100 of the presentembodiment, the pressure losses in the initial stage of the use of thehoneycomb filter and during the PM deposition can be reduced as comparedwith the conventional honeycomb filter 200 shown in FIG. 5. Here, “thesectional shape” is the shape in the cross section when the cells 2 ofthe honeycomb filter 100 shown in FIG. 1 to FIG. 4 are cut along a planeperpendicular to the central axis direction of the cells, and indicatesthe shape of each of portions surrounded with the partition walls 1forming the cells 2. Additionally, in the square and the rectanglementioned in the present specification, four corner portions 6 of eachof the square and the rectangle may form vertexes. In addition, the fourcorner portions 6 of each of the square and the rectangle may be formedinto a curved shape. When each of the corner portions 6 is formed intothe curved shape, a curvature radius is preferably 0.4 mm or less. Here,the corner portion 6 having a curvature radius of 0 mm is defined as thevertex.

Additionally, as shown in FIG. 2 and FIG. 3, in the honeycomb filter 100of the present embodiment, the plurality of cells 2 are arranged so thatthe periphery of one of the inlet plugged cells 2 c is surrounded witheight outlet plugged cells in the cross section perpendicular to thecentral axis direction of the honeycomb substrate 7. Here, the abovementioned eight outlet plugged cells are constituted of four squareoutlet plugged cells 2 b and four rectangular outlet plugged cells 2 a.Further, the cells are arranged so that each of four sides of the inletplugged cell 2 c having the square sectional shape in which the lengthof one side is L1 is adjacent to the long side of the rectangular outletplugged cell 2 a having the rectangular sectional shape in which thelength of the long side is L1 and the length of the short side is L2 andso that the adjacent sides are parallel to each other. Additionally, ineach diagonal direction of the inlet plugged cell 2 c having the squaresection shape in which the length of the one side is L1, four squareoutlet plugged cells 2 b each having the square section shape in whichthe length of one side is L2 are arranged adjacent to the inlet pluggedcell 2 c. In such a structure, the inlet plugged cells 2 c are notadjacent to each other, but the inlet plugged cell 2 c is surroundedwith four square outlet plugged cells 2 b and four rectangular outletplugged cells 2 a. According to such a structure, an open frontal areaof the inlet plugged cell 2 c can be enlarged, and the number of theinlet plugged cells 2 c can be smaller than the total number of therectangular outlet plugged cells 2 a and the square outlet plugged cells2 b, so that the initial pressure loss can be reduced.

In addition, as shown in FIG. 2 and FIG. 3, in a portion in which thevertexes of each of four cells 2 gather, i.e., a portion in which thefour vertexes (the corner portions 6) gather, the two partition walls 1are perpendicular to each other. It is to be noted that “the portion inwhich the four vertexes gather” is a portion in which one of thevertexes of the one inlet plugged cell 2 c, one of the vertexes of thesquare outlet plugged cell 2 b adjacent to the one inlet plugged cell 2c and one of the vertexes of each of the two rectangular outlet pluggedcells 2 a adjacent to the one inlet plugged cell 2 c gather. Accordingto such a structure, a high heat capacity of the partition walls 1 canbe maintained, and it is possible to alleviate the thermal stress duringthe PM burning in the vertex portion on which the PM is easilydeposited.

A distance between the partition wall 1 forming a first side 8 of theinlet plugged cell 2 c and the partition wall 1 forming a second side 9facing the first side 8 of the inlet plugged cell 2 c is defined as apartition wall center distance a. The partition wall center distance ais preferably in a range of 1.4 mm or more and 2.4 mm or less, furtherpreferably in a range of 1.4 mm or more and 2.2 mm or less, andespecially preferably in a range of 1.4 mm or more and 2.0 mm or less.Here, the partition wall center distance a indicates the shortestdistance that connects a center of a thickness direction of thepartition wall 1 forming the first side 8 of the inlet plugged cell 2 cto a center of a thickness direction of the partition wall 1 forming thefacing second side 9 of the inlet plugged cell 2 c. In addition, adistance between the partition wall 1 constituting a first long side 10of the rectangular outlet plugged cell 2 a and the partition wall 1forming a second long side 11 facing the first long side 10 of therectangular outlet plugged cell 2 a is defined as a partition wallcenter distance b. The partition wall center distance b is preferably ina range of 0.22 mm or more and 1.08 mm or less, further preferably in arange of 0.5 mm or more and 1.08 mm or less, and especially preferablyin a range of 0.8 mm or more and 1.08 mm or less. Here, the partitionwall center distance b indicates the shortest distance that connects acenter of a thickness direction of the partition wall 1 forming thefirst long side 10 of the rectangular outlet plugged cell 2 a to acenter of a thickness direction of the partition wall 1 forming thefacing second long side 11 of the rectangular outlet plugged cell 2 a. Arelation between the partition wall center distance a and the partitionwall center distance b is in the above range, whereby the initialpressure loss and the pressure loss during the PM deposition are reducedwith good balance. It is to be noted that each of the partition wallcenter distances a and b is a value measured by a method in which thecross section in the direction perpendicular to the central axisdirection of the honeycomb substrate 7 is observed with an opticalmicroscope.

The thickness t of the partition walls 1 is preferably from 0.16 mm to0.58 mm and further preferably from 0.16 mm to 0.40 mm .When thethickness t of the partition walls 1 is smaller than 0.16 mm, thestrength of the honeycomb substrate 7 deteriorates in a certain case.When the thickness t of the partition walls 1 is larger than 0.58 mm,the trapping performance deteriorates and the pressure loss enlarges ina certain case. It is to be noted that the thickness t of the partitionwalls is a value measured by the method in which the cross section inthe direction perpendicular to the axial direction of the honeycombsubstrate 7 is observed with the optical microscope.

According to the definitions of the respective partition wall centerdistances, the partition wall center distances a and b are “L1+t” and“L2+t”, respectively. As inner surface areas of the square outletplugged cells 2 b and the rectangular outlet plugged cells 2 a becomelarger, an area in which the PM is deposited enlarges, and a thicknessof the deposited PM during the PM deposition becomes smaller, so thatthe pressure loss during the PM deposition can be reduced. On the otherhand, when the inner surface areas of the square outlet plugged cells 2b and the rectangular outlet plugged cells 2 a are excessively large,the pressure loss during the PM deposition can be reduced, but theinitial pressure loss disadvantageously enlarges. In addition, as thepartition wall thickness t becomes smaller, the initial pressure lossand the pressure loss during the PM deposition decrease, but when thepartition wall thickness t is excessively small, the heat capacity ofthe honeycomb substrate excessively becomes small. In consequence, whenthe PM is burnt and removed, a temperature of the honeycomb substrateexcessively heightens, and the filter is damaged due to the excessivelylarge thermal stress in a certain case.

To satisfy both requirements, i.e., the reduction of the initialpressure loss and the pressure loss during the PM deposition and theprevention of the damage during the honeycomb filter regeneration, it isimportant that a value b/at obtained by dividing a product of thepartition wall center distance a and the partition wall thickness t bythe partition wall center distance b is larger than 0.95 and smallerthan 1.90. The above mentioned “b/at” is further preferably larger than1.20 and smaller than 1.90 and especially preferably larger than 1.40and smaller than 1.90.

There is not any special restriction on a manufacturing method of thehoneycomb filter 100 of the present embodiment. For example, thehoneycomb filter 100 can be manufactured by the following method. Amaterial selected from the above mentioned preferable materials, e.g.,silicon carbide (SiC) powder is used as raw material powder of thehoneycomb substrate 7, and a binder is added to this material.Furthermore, a surfactant and water are added, and a kneaded materialhaving plasticity is prepared. Examples of the binder includemethylcellulose and hydroxypropoxyl methylcellulose. This kneadedmaterial is extruded to obtain a formed body of the honeycomb substrate7 having the partition walls 1 and the cells 2 having such predeterminedsectional shapes as described above. This body is dried by, for example,microwaves and hot air, and then, plugging is performed with a materialsimilar to the material used in the extrusion of the honeycomb substrate7, whereby the plugging portions 3 are disposed in the formed body ofthe honeycomb substrate 7. Further, the formed body of the honeycombsubstrate 7 in which the plugging portions 3 are disposed is furtherdried, heated and degreased in, e.g., a nitrogen atmosphere, and thenfired in an inert atmosphere of argon or the like, so that the honeycombfilter 100 of the present embodiment can be obtained. A firingtemperature and a firing atmosphere differ with the raw material, and aperson skilled in the art can select the firing temperature and firingatmosphere which are optimum for the selected material.

An example of a method of obtaining the honeycomb filter 100 of thepresent embodiment as a constitution where the plurality of honeycombsegments are integrated is such a method as described below. Forexample, the plurality of honeycomb segments are bonded to one anotherby use of ceramic cement, and dried to harden, followed by processing acircumference into a desirable shape, so that the segment monolithictype of honeycomb filter 100 can be obtained.

In the honeycomb filter 100 of the present embodiment, a geometricsurface area GSA is preferably from 10 to 30 cm²/cm³ and furtherpreferably from 12 to 18 cm²/cm³ in the outlet plugged cells (the squareoutlet plugged cells 2 b and the rectangular outlet plugged cells 2 a).Here, the above mentioned “geometric surface area GSA” is a value (S/V)obtained by dividing a total inner surface area (S) of the outletplugged cells by a total volume (V) of the honeycomb substrate 7. Ingeneral, as a filtration area of the filter becomes larger, a thicknessof the PM deposited onto the partition walls can be reduced, so that thepressure loss can be minimized. When the geometric surface area GSA ofthe outlet plugged cells is smaller than 10 cm²/cm³, the pressure lossduring the PM deposition increases in a certain case. In addition, whenthe geometric surface area is larger than 30 cm²/cm³, the initialpressure loss increases in a certain case.

In the honeycomb filter 100 of the present embodiment, a cell crosssection open frontal area of the outlet plugged cells (the square outletplugged cells 2 b and the rectangular outlet plugged cells 2 a) ispreferably from 13 to 50% and further preferably from 14 to 42%. Whenthe cell cross section open frontal area of the outlet plugged cells issmaller than 13%, the initial pressure loss increases in a certain case.In addition, when the open frontal area is in excess of 50%, afiltration rate becomes fast, so that the trapping efficiency of the PMdeteriorates and further, the strength of the partition walls 1 becomesinsufficient in a certain case. Here, “the cell cross section openfrontal area of the outlet plugged cells” means a ratio of “the sum ofsectional areas of the outlet plugged cells” to the total of “a totalsectional area of the partition walls 1 forming the honeycomb substrate7” and “the sum of sectional areas of all the cells 2” in the crosssection perpendicular to the central axis direction of the honeycombsubstrate 7. It is to be noted that “the outlet plugged cell” is ageneral term for the square outlet plugged cell 2 b and the rectangularoutlet plugged cell 2 a. Therefore, “the sum of the sectional areas ofthe outlet plugged cells” is “the sum of the sectional areas of thesquare outlet plugged cells 2 b and the rectangular outlet plugged cells2 a”.

In view of trade-off among the initial pressure loss and the pressureloss during the PM deposition and the trapping efficiency, the honeycombfilter 100 of the present embodiment preferably has such a constitutionas described below. That is, it is preferable to simultaneously satisfythe requirements that the geometric surface area GSA of the outletplugged cells is from 10 to 30 cm²/cm³ and that the cell cross sectionopen frontal area of the outlet plugged cells is from 20 to 70%. Inaddition, it is further preferable to simultaneously satisfy therequirements that the geometric surface area GSA of the outlet pluggedcells is from 12 to 18 cm²/cm³ and that the cell cross section openfrontal area of the outlet plugged cells is from 25 to 65%.

In the plurality of cells 2, each of the corner portions 6 of the cells2 in the cross section perpendicular to the central axis direction ofthe honeycomb substrate 7 preferably has a curved shape having a radius.Here, “the corner portions 6” are corner portions described in thefollowing (1) to (3): (1) portions forming four corners in a squaresectional shape of the square outlet plugged cell 2 b in which thelength of one side is L2; (2) portions forming four corners in arectangular sectional shape of the rectangular outlet plugged cell 2 ain which the length of the long side is L1 and the length of the shortside is L2; and (3) portions forming four corners in a square sectionalshape of the inlet plugged cell 2 c in which the length of one side isL1. The corner portion 6 of the curved shape having the radiuspreferably has a curved shape in which a curvature radius is from 0.05to 0.4 mm, and further preferably has a curved shape in which acurvature radius is from 0.2 to 0.4 mm from the viewpoint of preventionof stress concentration. When the curvature radius of the corner portion6 is smaller than 0.05 mm, the PM is easily deposited on the cornerportions 6, and simultaneously, the thermal stress cannot be alleviatedand the strength of the partition walls 1 deteriorate, so that a thermalstress alleviation effect cannot sufficiently be produced in a certaincase. Additionally, when the curvature radius of the corner portion 6 islarger than 0.4 mm, the filtration area of the cells decreases in acertain case.

In the honeycomb filter 100 of the present embodiment, a catalyst (notshown) may be loaded onto the partition walls 1 forming the plurality ofcells 2. When the catalyst is loaded onto the partition walls 1, it ismeant that surfaces of the partition walls 1 and inner walls of poresformed in the partition walls 1 are coated with the catalyst. Examplesof a type of catalyst include SCR catalysts (zeolite, titania andvanadium) and a three-way catalyst including at least two of noblemetals such as Pt, Rh and Pd and at least one of alumina, ceria andzirconia. When such a catalyst is loaded onto the partition walls 1,NOx, CO, HC and the like included in the exhaust gas emitted from adirect injection type gasoline engine, a diesel engine or the like aredetoxified, and the PM deposited on the surfaces of the partition walls1 can easily be burnt and removed by a catalytic action.

There is not any special restriction on a method of loading such acatalyst as described above onto the honeycomb filter 100, and a methodusually performed by a person skilled in the art can be employed.Specifically, an example of the method is a method of carrying outwash-coating with a catalyst slurry, drying and firing.

EXAMPLES

Hereinafter, the present invention will further specifically bedescribed on the basis of examples, but the present invention is notlimited to these examples.

Example 1

As a ceramic raw material, a mixture of silicon carbide (SiC) powder andmetal silicon (Si) powder at a mass ratio of 80:20 was prepared. To thisceramic raw material, hydroxypropoxyl methylcellulose as a binder and awater absorbable resin as a pore former were added, and water was alsoadded, to prepare a forming raw material. The obtained forming rawmaterial was kneaded by using a kneader, to obtain a kneaded material.

Next, the obtained kneaded material was formed by using a vacuumextruder and 16 rectangular pillar-shaped honeycomb segments each havingsuch a plugging arrangement as shown in FIG. 2 and FIG. 3 were prepared.A sectional shape of each honeycomb segment in a direction perpendicularto a cell extending direction was a square of 36 mm×36 mm and thesegment had a length of 152 mm. In addition, a partition wall centerdistance a shown in FIG. 2 was set to 2.2 mm, a partition wall centerdistance b was set to 0.76 mm and a partition wall thickness t was 0.30mm.

Subsequently, the obtained honeycomb segments were subjected to highfrequency induction heating drying and then dried at 120° C. for 2 hoursby use of a hot air drier. Additionally, during the drying, thehoneycomb segments were arranged so that outflow end faces wereperpendicularly directed downwardly, to carry out the drying.

In the dried honeycomb segments, plugging portions were formed. First, amask was applied to an inflow end face of each honeycomb segment. Next,each masked end face (an outflow end face) was immersed into a pluggingslurry to charge the plugging slurry into open ends of cells (inletplugged cells) which were not masked, and the plugging portions (inflowside plugging portions) were formed and dried. Further, also in theoutflow end face of the dried honeycomb segment, plugging portions(outflow side plugging portions) were similarly formed in the residualcells (i.e., the cells which were not plugged in the inflow end face(rectangular outlet plugged cells and square outlet plugged cells)).

Further, the honeycomb segments in which the plugging portions wereformed were degreased and fired, and the plugged honeycomb segments wereobtained. As degreasing conditions, the degreasing was carried out at550° C. for 3 hours, and as firing conditions, the firing was carriedout at 1450° C. under an argon atmosphere for 2 hours. Additionally,during the firing, the honeycomb segments were arranged so that outflowend faces 5 b were perpendicularly directed downwardly, to carry out thefiring.

The 16 fired honeycomb segments were bonded and integrated by using abonding material (ceramic cement). The bonding material was prepared byusing inorganic particles and an inorganic adhesive as main componentsand including an organic binder, a surfactant, a foamable resin, waterand the like as subcomponents. As the inorganic particles, plate-shapedparticles were used, and as the inorganic adhesive, colloidal silica(silica sol) was used. As the plate-shaped particles, mica was used. Acircumference of a honeycomb segment bonded body in which the 16honeycomb segments were integrally bonded was ground and processed intoa round pillar shape, and a coating material was applied to acircumferential surface of the bonded body to obtain a completed body (ahoneycomb filter). The coating material was prepared to include ceramicpowder, water and a bonding agent.

By the above steps, a honeycomb filter of Example 1 of such a pluggingarrangement as shown in FIG. 2 and FIG. 3 was prepared. Table 1 shows,as X, the plugging arrangement in which one inlet plugged cell issurrounded with eight outlet plugged cells as shown in FIG. 2 and FIG.3.

Examples 2 to 15 and Comparative Examples 1 to 12

The procedure of Example 1 was repeated except that a partition wallcenter distance a, a partition wall center distance b and a partitionwall thickness t were set to values shown in Table 1, to preparehoneycomb filters of Examples 2 to 15 and Comparative Examples 1 to 12.

Comparative Examples 13 and 14

The procedure of Example 1 was repeated except that plugging portionswere arranged in a checkered manner and a partition wall center distancea, a partition wall center distance b and a partition wall thickness twere set to values shown in Table 1, to prepare honeycomb filters ofComparative Examples 13 and 14 having such a plugging arrangement asshown in FIG. 5. Table 1 shows, as Y, the plugging arrangement in whichinlet plugged cells and outlet plugged cells are arranged in thecheckered manner as shown in FIG. 5.

Concerning each of the honeycomb filters of Examples 1 to 15 andComparative Examples 1 to 14, an initial pressure loss, a pressure lossduring PM deposition, a regeneration efficiency during forcedregeneration and a continuous regeneration soot amount in an NEDC modeoperation were measured and evaluated. Table 1 shows the results.

(Measuring Method of Initial Pressure Loss)

Air at 200° C. was allowed to flow through the honeycomb filter at 2.4Nm³/min to measure a pressure loss in an initial stage (the initialpressure loss (kPa)) from a pressure difference between an inflow sideand an outflow side. Here, a heat capacity is a function of a cell crosssection open frontal area (OFA) and is proportional to (1-OFA).Therefore, the larger the OFA is, the smaller the heat capacity becomes,and hence, cracks are easily generated during honeycomb filterregeneration. As an index which satisfies both of requirements that theinitial pressure loss is low and that the cracks are hard to begenerated during the honeycomb filter regeneration, α=(the initialpressure loss)×(OFA)² is used. It can be judged that, as α is smaller,the honeycomb filter is excellent in performance. Specifically, it isjudged that when α is 0.60 or more, the honeycomb filter fails, and whenα is smaller than 0.60, the honeycomb filter passes.

(Measuring Method of Pressure Loss During PM Deposition)

Light oil was burnt in a state where oxygen was running short, togenerate soot, and dilution air was added to a combustion gas having asoot generation amount of 10 g/h, whereby the soot-containing combustiongas regulated and allowed to flow through the honeycomb filter at atemperature of 200° C. and a flow rate of 2.4 Nm³/min. Further, thepressure loss during the PM deposition (the PM deposition pressure loss(kPa)) was measured from the pressure difference between the inflow sideand the outflow side when the amount of the soot deposited onto thehoneycomb filter was 4 g/L. It is to be noted that, as described above,the heat capacity is the function of the cell cross section open frontalarea (OFA) and is proportional to (1-OFA). Therefore, the lager the OFAis, the smaller the heat capacity becomes, and hence, the cracks areeasily generated during the honeycomb filter regeneration. As an indexwhich satisfies both of requirements that the PM deposition pressureloss is low and that the cracks are hard to be generated during thehoneycomb filter regeneration, β=(the PM deposition pressureloss)×(OFA)² is used. It can be judged that, as β is smaller, thehoneycomb filter is excellent in performance. Specifically, it is judgedthat when β is 3.70 or more, the honeycomb filter fails, and when β issmaller than 3.70, the honeycomb filter passes.

(Regeneration Efficiency During Forced Regeneration)

In a state where 6 g/L of the soot was deposited onto the partitionwalls of the honeycomb filter, a high-temperature gas was passed from aninflow end face of the honeycomb filter, to carry out forcedregeneration of the filter. For conditions of the forced regeneration,the temperature of the gas in the inflow end face was set to 650° C. anda passing time of the gas was set to 15 minutes. In addition, a mass ofthe honeycomb filter in which the soot was deposited was measured priorto the forced regeneration. After the forced regeneration, the mass ofthe honeycomb filter was measured, and a mass of the soot lost by theforced regeneration was obtained. A regeneration efficiency (M2/M1×100)during the forced regeneration was obtained from a mass M1 of thedeposited soot and a mass M2 of the soot lost by the forcedregeneration. Table 1 shows the regeneration efficiency during theforced regeneration as “the regeneration efficiency (%)”.

(Continuous Regeneration Soot Amount in NEDC Mode Operation)

Concerning the honeycomb filter in a state where 6 g/L of the soot wasdeposited onto the partition walls of the honeycomb filter, a continuousregeneration soot amount was measured under the NEDC (New EuropeanDriving Cycle) mode operation by installing a 4-cylinder 2000 cc dieselengine car of common rail injection in a chassis bench. The NEDC modeoperation is an operation under conditions of a car speed pattern inconformity with European regulations. Additionally, a mass of thehoneycomb filter in which the soot was deposited was measured prior tothe NEDC mode operation. After the NEDC mode operation, the mass of thehoneycomb filter was measured, and a mass of the soot burnt and lost inthe NEDC mode operation was obtained. From a mass M3 of the depositedsoot and a mass M4 of the soot burnt and lost by the NEDC modeoperation, the continuous regeneration soot amount (M4−M3) in the NEDCmode operation was obtained. Table 1 shows the continuous regenerationsoot amount in the NEDC mode operation as “the continuous regenerationsoot amount (g/L)”.

(General Evaluation)

From the measured pressure loss in the initial stage, the measuredpressure loss during the PM deposition, the measured regenerationefficiency during the forced regeneration and the measured continuousregeneration soot amount in the NEDC mode operation, evaluations ofexcellent, good and failure were carried out in accordance with thefollowing judgment standards.

-   Evaluation “excellent”: a case where the regeneration efficiency    during the forced regeneration is 70% or more, the continuous    regeneration soot amount in the NEDC mode operation is 1.0 (g/L) or    more, a value of α is smaller than 0.60 and a value of β is smaller    than 3.67.-   Evaluation “good”: a case where the regeneration efficiency during    the forced regeneration is 70% or more, the continuous regeneration    soot amount in the NEDC mode operation is 1.0 (g/L) or more, a value    of α is smaller than 0.60 and a value of β is smaller than 3.70.    Evaluation “failure”: a case where the regeneration efficiency    during the forced regeneration is smaller than 70%, or the    continuous regeneration soot amount in the NEDC mode operation is    smaller than 1.0 (g/L), or a value of α is 0.60 or more or a value    of β is 3.70 or more.

TABLE 1 Plug- Initial PM Regen- Continuous ging pres- depo- erationregen- ar- sure sition effi- eration range- loss pressure ciency sootamount General a b t b/a b/at ment OFA (kPa) loss (kPa) α β % (g/L)judgment Comp. 2.3 0.60 0.30 0.26 0.88 X 0.64 0.79 9.30 0.33 3.81 82 1.7Failure Ex 1 Example 1 2.2 0.66 0.30 0.30 1.01 X 0.64 0.82 8.98 0.343.68 82 1.7 Good Example 2 2.1 0.71 0.30 0.34 1.13 X 0.64 0.86 8.82 0.353.61 82 1.7 Excellent Example 3 1.7 0.89 0.30 0.52 1.74 X 0.64 1.20 8.560.49 3.50 81 1.7 Excellent Comp. 1.6 0.92 0.30 0.58 1.92 X 0.64 1.468.59 0.60 3.52 81 1.7 Failure Ex 2 Comp. 2.4 0.67 0.32 0.28 0.88 X 0.640.75 9.26 0.31 3.79 80 1.3 Failure Ex 3 Example 4 2.3 0.73 0.32 0.320.99 X 0.64 0.79 8.94 0.32 3.66 81 1.3 Excellent Example 5 2.2 0.78 0.320.35 1.11 X 0.64 0.83 8.80 0.34 3.61 80 1.3 Excellent Example 6 1.7 0.980.32 0.58 1.80 X 0.64 1.25 8.65 0.51 3.54 80 1.3 Excellent Comp. 1.61.02 0.32 0.64 1.99 X 0.64 1.47 8.73 0.60 3.58 79 1.3 Failure Ex 4 Comp.2.5 0.75 0.34 0.30 0.88 X 0.64 0.72 9.10 0.29 3.73 78 1.1 Failure Ex 5Example 7 2.4 0.80 0.34 0.33 0.98 X 0.64 0.76 8.89 0.31 3.64 78 1.1Excellent Example 8 2.3 0.85 0.34 0.37 1.08 X 0.64 0.80 8.79 0.33 3.6077 1.1 Excellent Example 9 1.7 1.08 0.34 0.63 1.86 X 0.64 1.28 8.78 0.523.60 78 1.1 Excellent Comp. 1.6 1.10 0.34 0.69 2.03 X 0.64 1.50 8.900.61 3.65 78 1.1 Failure Ex 6 Comp. 2.1 0.27 0.16 0.13 0.80 X 0.77 0.886.60 0.52 3.91 82 2.1 Failure Ex 7 Exam- 2.0 0.34 0.16 0.17 1.05 X 0.770.86 6.23 0.51 3.69 82 2.1 Good ple 10 Exam- 1.7 0.49 0.16 0.29 1.79 X0.77 0.90 5.59 0.53 3.32 82 2.1 Excellent ple 11 Comp. 1.6 0.52 0.160.33 2.04 X 0.77 1.01 5.44 0.60 3.23 82 2.1 Failure Ex 8 Comp. 2.0 0.340.18 0.17 0.93 X 0.77 0.90 6.60 0.53 3.91 81 2.0 Failure Ex 9 Exam- 1.90.40 0.18 0.21 1.16 X 0.77 0.90 6.18 0.53 3.66 81 2.0 Excellent ple 12Exam- 1.6 0.52 0.18 0.33 1.81 X 0.77 0.91 5.63 0.54 3.34 80 2.0Excellent ple 13 Comp. 1.5 0.56 0.18 0.37 2.06 X 0.77 1.08 5.50 0.643.26 80 2.0 Failure Ex 10 Comp. 2.0 0.34 0.20 0.17 0.84 X 0.77 0.91 6.690.54 3.97 79 1.8 Failure Ex 11 Exam- 1.9 0.40 0.20 0.21 1.04 X 0.77 0.916.23 0.54 3.69 79 1.8 Good ple 14 Exam- 1.5 0.56 0.20 0.37 1.86 X 0.770.95 5.71 0.56 3.38 79 1.8 Excellent ple 15 Comp. 1.4 0.59 0.20 0.422.11 X 0.77 1.21 5.61 0.72 3.33 79 1.8 Failure Ex 12 Comp. 2.1 0.71 0.300.34 1.13 Y 0.64 0.86 8.82 0.35 3.61 66 0.3 Failure Ex 13 Comp. 1.471.47 0.30 1.00 3.33 Y 0.64 1.95 10.25 0.80 4.20 61 0.2 Failure Ex 14

(Result)

It has been seen from Table 1 that, as compared with a conventionalfilter in which plugging portions are arranged in a checkered manner,honeycomb filters of Examples 1 to 15 each having a cell sectionalstructure shown in FIG. 2 and FIG. 3 indicate suitable results in anyone of an initial pressure loss, a pressure loss during PM deposition, aregeneration efficiency during forced regeneration and a continuousregeneration soot amount in an NEDC mode operation. In addition, it hasbeen seen that, in a case where a value b/at obtained by dividing aproduct of a partition wall center distance a and a partition wallthickness t by a partition wall center distance b is larger than 0.95and smaller than 1.90, more significant effects are produced in any oneof the initial pressure loss and the pressure loss during the PMdeposition, as compared with the other cases.

A honeycomb filter according to the present invention is suitably usableas a DPF for use in purification of particulates and toxic gascomponents included in an exhaust gas emitted from a direct injectiontype gasoline engine, a diesel engine or the like.

DESCRIPTION OF REFERENCE NUMERALS

1: partition wall, 2: cell, 2 a: rectangular outlet plugged cell, 2 b:square outlet plugged cell, 2 c: inlet plugged cell, 3: pluggingportion, 3 a: inflow side plugging portion, 3 b: outflow side pluggingportion, 4: side, 5 a: inflow end face, 5 b: outflow end face, 6: cornerportion, 7: honeycomb substrate, 8: first side of the inlet pluggedcell, 9: second side of the inlet plugged cell, 10: first long side ofthe rectangular outlet plugged cell, 11: second long side of therectangular outlet plugged cell, 100: honeycomb filter, 200: honeycombfilter, 201: partition wall, 202: cell, 202 a: outlet plugged cell, 202b and 202 c: inlet plugged cell, 203: plugging portion, 205 a: inflowend face, a: partition wall center distance, b: partition wall centerdistance, and t: partition wall thickness.

What is claimed is:
 1. A honeycomb filter comprising: a honeycombsubstrate having porous partition walls defining a plurality of cellswhich extend from an inflow end face as an end face on an exhaust gasinflow side to an outflow end face as an end face on an exhaust gasoutflow side and which become through channels for a fluid; and pluggingportions disposed in end portions of the plurality of cells on one of aninflow end face side and an outflow end face side, wherein the cells ofpart of the plurality of cells are inlet plugged cells whose endportions are closed by the plugging portions on the inflow end face sideof the honeycomb substrate, and the residual cells among the pluralityof cells are outlet plugged cells whose end portions are closed by theplugging portions on the outflow end face side of the honeycombsubstrate, wherein the plurality of cells are arranged so that aperiphery of one of the inlet plugged cells is surrounded with eight ofthe outlet plugged cells in a cross section perpendicular to a centralaxis direction of the honeycomb substrate, a shape of an open end ofeach of the inlet plugged cells in the cross section is a square inwhich a length of one side is L1, the outlet plugged cells includesquare outlet plugged cells and rectangular outlet plugged cells, ashape of an open end of each of the square outlet plugged cells in thecross section is a square in which a length of one side is L2, and L2 issmaller than L1, a shape of an open end of each of the rectangularoutlet plugged cells in the cross section is a rectangle in which alength of a long side is L1 and a length of a short side is L2, in eachdiagonal direction of the square in which the length of the one side isL1 in the cross section, four of the square outlet plugged cells arearranged adjacent to the inlet plugged cell, in a linear directionperpendicular to each side of the square in which the length of the oneside is L1 in the cross section, four of the rectangular outlet pluggedcells are arranged so that the cells are adjacent to the inlet pluggedcell and so that the long side of the rectangle in the cross section isparallel to one side of the square in which the length of the one sideis L1 in the cross section, a partition wall thickness of the partitionwalls is defined as t, a distance from an intermediate point of thethickness of the partition wall defining one side of the open end of thesquare in which the length of the one side is L1 in the cross section toan intermediate point of the thickness of the partition wall defining aside facing the one side is defined as a partition wall center distancea, a distance from an intermediate point of the thickness of thepartition wall defining a long side of the open end of the rectangle inthe cross section to an intermediate point of the thickness of thepartition wall defining a side facing the long side is defined as apartition wall center distance b, and the partition wall center distancea, the partition wall center distance b and the partition wall thicknesst satisfy the following equation (1),0.95<b/at<1.90   (1).
 2. The honeycomb filter according to claim 1,wherein the partition wall center distance a is 1.4 mm or more and 2.4mm or less.
 3. The honeycomb filter according to claim 1, wherein thepartition wall center distance b is 0.22 mm or more and 1.08 mm or less.4. The honeycomb filter according to claim 2, wherein the partition wallcenter distance b is 0.22 mm or more and 1.08 mm or less.
 5. Thehoneycomb filter according to claim 1, wherein the partition wallthickness t is 0.16 mm or more and 0.34 mm or less.
 6. The honeycombfilter according to claim 2, wherein the partition wall thickness t is0.16 mm or more and 0.34 mm or less.
 7. The honeycomb filter accordingto claim 3, wherein the partition wall thickness t is 0.16 mm or moreand 0.34 mm or less.
 8. The honeycomb filter according to claim 4,wherein the partition wall thickness t is 0.16 mm or more and 0.34 mm orless.
 9. The honeycomb filter according to claim 1, wherein a celldensity of the honeycomb substrate is from 100 to 650 cells/cm².
 10. Thehoneycomb filter according to claim 1, wherein an open frontal area ofthe outlet plugged cells on the inflow end face side is from 13 to 50%.11. The honeycomb filter according to claim 1, wherein a porosity of thepartition walls is from 28 to 70%.